Tracking issue for retain() method for HashMap (`retain_hash_collection`)

[navaati]

The HashMap type does not have a retain method like the one in Vec.

[Cobrand]

I wonder if that needs an RFC ? Pretty small change to std, but still a change to std

[nagisa]

Small backward-compatible additions do not need an RFC unless they hit some sort of controversy.

[mbrubeck]

This was added in nightly as an unstable feature retain_hash_collection with a link to this issue. The issue should be re-opened for tracking stabilization. CC @rust-lang/libs